In a chemical reaction, oxidation always occurs with an accompanying reduction reaction. The oxidation reaction and the reduction are in fact referred to as half reactions because they together form a full reaction.
Oxidation-reduction reactions are more commonly referred to as redox reactions. One view of redox reactions, which is the earliest, is in terms of oxygen transfer: oxidation is the gain of oxygen, while reduction is loss of oxygen. For example, in the combustion of hydrogen, oxygen is reduced because it loses one oxygen atom, and hydrogen is oxidized because it gains an oxygen atom, forming water.
An alternative view of redox reactions is in terms of gain and loss of hydrogen: oxidation is the loss of hydrogen, while reduction is the gain of hydrogen. For example, in the complete combustion of methane, carbon is oxidized because it loses four hydrogen atoms, and oxygen is reduced because it gains two hydrogen atoms to form water.
A more scientifically accepted view of redox reactions is in terms of loss and gain of electrons: oxidation is the loss of electrons, while reduction is the gain of electrons. For example, in the reaction between magnesium and chlorine, magnesium is oxidized because it loses two electrons, and chlorine is reduced because it gains two electrons.